Autistic employees frequently face significant challenges in social interaction, communication, time and task management, and sensory perception. Assistive technologies (AT) and tailored customization can help overcome these barriers and improve workplace conditions for autistic individuals. However, the effective design of AT requires an in-depth understanding of the heterogeneous needs and independent influencing factors of this population. This paper presents a framework to support the development of authentic personas, which serve as a foundation for designing AT that meets the specific needs of autistic employees. We conducted a comprehensive context analysis by observing real training scenarios in a vocational training center and supplemented these observations with in-depth interviews with autistic individuals and diverse stakeholders. Drawing on these data and guided by the International Classification of Functioning, Disability and Health (ICF), we defined meaningful personal and environmental attributes that underpin our persona development. The resulting five realistic personas capture a wide range of traits and challenges that critically impact the work experience of autistic employees in office environments. This approach balances the diversity of the target group while providing actionable insights into their support needs. Finally, the paper discusses the limitations and potentials of using personas in the design of AT.